System for maintaining constant the dry material flow to a sheet material manufacturing machine

ABSTRACT

The stockline valve of a paper making machine is regulated by comparing by means of a digital computer successive readings of the consistency of the paper pulp carried by a water carrier. Any resultant difference between such readings is converted to analog information and coupled to the set point of an analog flow control unit which is responsive to the actual flow rate to control the stock valve. The digital computer will perform changes of the flow controller set point in such a manner that the product of consistency and flow set point is constant. This provides control of the pulp flow in the line.

O United States Patent [151 3,650,891 Dahlin 1 Mar. 21, 1972 54] SYSTEMFOR MAINTAINING 3,490,689 1/1970 Hart et a1. ..162/252 x D CONSTANT THERY MATERIAL OTHER PUBLICATIONS FLOW TO A SHEET MATERIAL MANUFACTURINGMACHINE Perry s Chemical Engineer s Handbook pp. 22: 5 l- 56 & 63-

64 [72] Inventor: Erik B. Dahlin, Saratoga, Calif. Amber et a1.,Automatic Control, May 1958 pp. 43, 45- 47 [73] Assignee: MeasurexCorporation, Santa Clara, Calif. ggi g Convemlon Record 1965 03 90. 94 &[22] Filed: Apr. 7, 1969 Primary Examiner-S. Leon Bashore PP ,100Assistant Examiner-Thomas G. Scavone Attorney-Flehr, Hohbach, Test,Albritton & Herbert [52] US. Cl ..l62/259, 162/263, 162/380,

235/151.1, 235/I51.34, 235/15135 [57] ABSTRACT [51 Int. Cl. ..D2lf 1/06The stockline valve of a paper making machine is regulated by 58] Fieldof Search 1 62/258, 259, 380, 252, 263; compa ng by means of a digitalcomputer successive readings 37 2 4 75; 235/1513 15134 15131 511 oftheconsistency ofthe paper pulp carried by a water carrier. 15112 Anyresultant difference between such readings is converted to analoginformation and coupled to the set point of an [56] References Citedanalog flow control unit which is responsive to the actual flow rate tocontrol the stock valve. The digital computer will per- UNITED STATESPATENTS form changes of the flow controller set point in such a mannerthat the product of consistency and flow set point is constant.1,123,124 11/1914 HBZI'COUII 1 i p i control ofthe p p flow in the line.2,379,746 7/1945 Poirier 3,129,323 4/1964 Stout et al ..235/I5I.l2 X 5Claims, 1 Drawing Figure 10 l PAPER HEAD BOX I6 WEB 2 FLOW CONSISTENCY FI Q I ZI ANALOG 28 FLOW CHANGE IN CONTROL SECONDARY SET POINT MANUAL D AA D OVERRIDE CONV CONV.

DIGITAL I COMPUTER 26 DRY STOCK I 24 FLOW EVALUATION T CONSISTENCY IPRIMARY COMMAND I I SET POINT STAT'ON DIGITAL w I CONTROLLER 22 [I [I] Ll NOMINAL ON LINE DRY STOCK FLOW SET POINT PATENTEDHARZI I972 3650.891

' HEAD PAPER 50x l6 WEB I2 FLOW H CONSISTENCY o F 2c? A 2| ANALOG 28FLOW CHANGE IN CONTROL SECONDARY /SET POINT 3| MANUAL D/A A/D OVERRIDECONV. CONV DIGITAL COMPUTER 26 DRY STOCK FLOW I EVALUATION TCONSIISTENCY xl PRIMARY COMMAND SET POINT STAT'ON DIGITAL CONTROLLER I22 E] U I...

NOMINAL ON LINE DRY STOCK FLOW SET POINT INVENTOR.

ERIK B. DAHLIN BY #M, Mafia, ML, W

ATTORNEYS SYSTEM FOR MAINTAINING CONSTANT THE DRY MATERIAL FLOW TO ASHEET MATERIAL MANUFACTURING MACHINE BACKGROUND OF THE INVENTION Thepresent invention is directed to a system for maintaining constant thedry material flow to a sheet material manufacturing machine and moreparticularly to a system where the sheet material is paper.

In a paper making process it is desirable to maintain the paper pulpflow in the stockline to the headbox constant. Variations can occureither in the flow rate, expressed as volume per unit time of the liquidcarrying the paper fiber, or in consistency which is the amount orvolume of pulp or paper fiber carried by a unit volume of water. Theproduct of flow rate and consistency is, of course, the actual pulp ordry material flow which it isdesired to maintain constant.

In the past, flow rate and consistency have been measured by twoseparate metering devices and the flow rate adjusted, either manually orby computer, to compensate for minor variations in consistency. Where acomputer of the digital type was used it was necessary to store bothflow rate and consistency information, compare the product to areference standard and thereafter make the proper control adjustment inthe flow rate. The flow rate is normally the only variable controlledfor small changes in consistency. An increase in flow rate, for example,provides more paper fiber thus compensat-' ing for a lower consistencyand vice versa,

While the foregoing computer control is adequate, it requires arelatively large amount of computer storage and time, increasingcomputer costs, and is not readily adaptable to optional manualoverride. This may be necessary if one of the metering unitsmalfunctions.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, a general objectof the invention to provide an improved system for maintaining constantthe dry material flow in a process for manufacturing sheet material.

It is another object of the invention to provide a system as above whichis automated but requires a minimum of computer storage and time and isreadily adaptable to manual override.

In accordance with the above objects there is provided a system formaintaining constant the dry material flow to a machine formanufacturing sheet material where the dry material flow is the productof the flow rate of the material suspended in a liquid carrier beingsupplied to the machine and the consistency of the material relative tothe liquid carrier, but where only said flow rate is controlled. Thesystem includes means for measuring the consistency and digital meansfor comparing the difference between successive measurements ofconsistency. Means are provided for measuring the flow rate and forcontrolling the flow rate. Analog control means are responsive to thechanges in consistency and the measured flow rate to adjust the flowrate in inverse proportion to the consistency change whereby the drymaterial flow is maintained constant.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a block diagramshowing a system embodying the present invention in conjunction with apartial schematic representation of a machine for manufacturing sheetmaterial.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated in thedrawing, the present invention is directed to a system for controllingthe dry stock flow being fed to the headbox of a paper making machine.However, it is applicable to other similar types of sheet materialmanufacturing machines. The dry stock material composed of paper pulp iscarried by water in stockline 11 through a control valve 12. The paperpulp in stockline 11 is metered by both a flow meter 13 which measuresvolume per unit time and a consistency meter 14 which measures volume orweight of the paper pulp or stock material per unit weight of thecarrier such as-water.

Headbox 10 includes a slicer unit 16 which produces paper web 17. Thisis the initial formation of the final paper or sheet material output ofthe paper making machine. Web 17 is formed on a wire mesh 18 mounted onrollers 19. Excess paper pulp is captured by trough 21 and recirculatedinto headbox 10. The remainder of the paper making machine is not shownbut includes suitable transport means, rollers and a steam system.

In addition, the paper making machine includes measuring instruments(not shown) for measuring both the moisture content of the paper beingproduced and its basis weight which is the weight per unit area. Suchmeasuring devices are disclosed in copending applications entitledApparatus for Measuring the Amount of a Substance That Is AssociatedWith a Base Material in the names of Hill et al., Ser. No. 799,784,filed Feb. 17, 1969 and Basis Weight Gauging Apparatus, System andMethod" in the names of Bossen et al., Ser. No. 791,972, filed Jan. 17,1969, and now abandoned.

As described in the above copending applications, basis weightmeasurements are coupled to a digital controller 22 which automaticallyadjusts the dry stock flow set point. In response to desired basisweight values, the digital controller determines what desired rate ofdry material should be supplied to headbox 10. In the case of a paper,making machine this would be the amount of dry paper pulp or stock whichit is desired to be supplied to headbox 10 for any unit increment oftime. This information designated primary set point is coupled to thedry stock or material flow evaluation unit 23.

Both digital controller 22 and dry stock flow evaluation unit 23 arepart of a digital computer unit 24 which serves the overall purposes ofthe paper making machine control system. Also coupled to digitalcomputer 24 is a consistency command station 26 which has two pushbuttons designated nominal and on line. The on line push buttoncompletes the communication link between consistency meter 14 and thedry stock flow evaluation unit 23 through analog to digital converter 27toform a control loop which will be discussed in detail below. Thenominal push button is depressed in the event of a failure ormalfunction of consistency meter 14. This removes the consistency meterfrom the control loop and in its place a nominal consistency value isstored in the dry stock flow evaluation unit 23. This allows the controlsystem to remain operative even if the consistency meter 14 fails.

The digital computer means 24 which includes dry stock evaluation unit23 compares successive measurements of consistency from meter 14 andproduces an error signal proportional to the change of consistency. Thiserror signal is coupled to an analog fiow control unit 28 through adigital to analog converter 29. Such error signal is designated changein secondary set point. Analog flow control unit 29 is coupled to flowmeter 13 and adjusts the stock valve 12 in response to any error signal.This adjustment is, of course, inversely proportional to the change ofconsistency in order to maintain dry stock flow constant.

In operation, computer 24 reads the signal from consistency meter 14repetitively. For example, readings may be made every two seconds and anaverage based on four samples is suitable for the system. This averagecan be achieved, for example, by filtering the four samples by means ofa filtering algorithm to eliminate high frequency noise. Dry stock flowevaluation unit 23 calculates a change in the secondary set point byASSP== -PSP X AC Constant 1. where ASSP" is the change of the secondaryset point, PSP" the primary set point which is the desired dry stock orpaper pulp flow in stockline 11, AC is the change in the reading of theconsistency from a previous control cycle and the constant is forpurposes of conversion of units.

It is apparent from the above equation that during the start of thesystem the secondary set point is zeroed in or offset by use of analogflow control unit 28 so that in effect a zero error signal will beproduced by dry stock flow evaluation unit 23 when the system is workingat the desired values of consistency and flow rate. Thus, in accordancewith the equation, when a change occurs in the consistency or density ofthe paper pulp and its liquid water carrier, the flow rate can then beadjusted to compensate accordingly by, for example, increasing the flowwhich provides the flow of more paper pulp or vice versa.

A manual override unit 31 coupled to digital to analog converter 29allows easy manual operator control of the flow rate by use of theanalog flow control unit 28. The manual override 31 is completelyindependent of the digital computer system 24 and thus permits operationwithout a digital computer.

The system of the present invention by use of an analog control loopincluding analog flow control unit 28, flow meter 13 and stock valve 12eliminates a large load on computer 24 which would normally be the caseif all of the functions of the flow control unit were to be digitallycontrolled. This is especially true in view of the large number ofreadings which must be taken. In addition, since digital controller 22performs many other functions in the paper making system the presentinvention reduces problems of time sharing computer logic. Lastly,analog control unit 28 is readily adaptable to manual control which isfrequently necessary in a system of the present type.

What is claimed is:

l. A system for maintaining constant the dry material flow to a machinefor manufacturing sheet material where said dry material flow is theproduct of the flow rate of the material suspended in a liquid carrierbeing supplied the machine and the consistency of the material relativeto the liquid carrier but where only said flow rate is controlled, thesystem comprising means for measuring said consistency, digital meansfor comparing successive readings of said measured consistency andproducing an error signal proportional to any lack of comparison, meansfor measuring said flow rate, means for controlling said flow rate, andanalog control means, responsive to said error signal and said measuredflow rate for adjusting said flow rate in proportion to said errorsignal whereby said dry material flow is maintained constant.

2. A system as in claim 1 together with means for manually overridingsaid error signal whereby said analog control means may be manuallycontrolled.

3. A system as in claim 1 together with analog to digital convertermeans for coupling said consistency measuring means to said digitalmeans and digital to analog converting means for coupling said errorsignal from said digital means to said analog control means.

4. A system as in claim 1 together with means for setting said measuredconsistency to a nominal value whereby said system is immunized againsta malfunction in said consistency measuring means.

5. A system as in claim 1 in which said digital means causes said errorsignal to very in inverse proportion to said change in consistency inaccordance with the equation ASSP=PSP AC constant where A? is saidvariation of error signal, PSP is a primary set point which is anindependent variable indicative of desired dry material flow and AC ischange in consistency, whereby said flow rate is adjusted in inverseproportion to said change in consistency.

1. A system for maintaining constant the dry material flow to a machinefor manufacturing sheet material where said dry material flow is theproduct of the flow rate of the material suspended in a liquid carrierbeing supplied the machine and the consistency of the material relativeto the liquid carrier but where only said flow rate is controlled, thesystem comprising means for measuring said consistency, digital meansfor comparing successive readings of said measured consistency andproducing an error signal proportional to any lack of comparison, meansfor measuring said flow rate, means for controlling said flow rate, andanalog control means, responsive to said error signal and said measuredflow rate for adjusting said flow rate in proportion to said errorsignal whereby said dry material flow is maintained constant.
 2. Asystem as in claim 1 together with means for manuallY overriding saiderror signal whereby said analog control means may be manuallycontrolled.
 3. A system as in claim 1 together with analog to digitalconverter means for coupling said consistency measuring means to saiddigital means and digital to analog converting means for coupling saiderror signal from said digital means to said analog control means.
 4. Asystem as in claim 1 together with means for setting said measuredconsistency to a nominal value whereby said system is immunized againsta malfunction in said consistency measuring means.
 5. A system as inclaim 1 in which said digital means causes said error signal to very ininverse proportion to said change in consistency in accordance with theequation Delta SSP -PSP X Delta C X constant where Delta SSP is saidvariation of error signal, PSP is a primary set point which is anindependent variable indicative of desired dry material flow and Delta Cis change in consistency, whereby said flow rate is adjusted in inverseproportion to said change in consistency.